linux-kernel - Re: [PATCH v3 2/3] x86/smp native_play_dead: Prefer cpuidle_play_dead() over mwait_play

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Message-ID: <CAJZ5v0hJ8NoFgjtnYce99+qjCZc3_ihBojyK1gRrcyU5Fp6inw@mail.gmail.com>
Date: Tue, 12 Nov 2024 14:23:14 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>, x86@...nel.org, 
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, 
	rafael.j.wysocki@...el.com, len.brown@...el.com, 
	artem.bityutskiy@...ux.intel.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH v3 2/3] x86/smp native_play_dead: Prefer
 cpuidle_play_dead() over mwait_play_dead()

On Tue, Nov 12, 2024 at 12:47 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Nov 08, 2024 at 01:29:08PM +0100, Patryk Wlazlyn wrote:
> > The generic implementation, based on cpuid leaf 0x5, for looking up the
> > mwait hint for the deepest cstate, depends on them to be continuous in
> > range [0, NUM_SUBSTATES-1]. While that is correct on most Intel x86
> > platforms, it is not architectural and may not result in reaching the
> > most optimized idle state on some of them.
> >
> > Prefer cpuidle_play_dead() over the generic mwait_play_dead() loop and
> > fallback to the later in case of missing enter_dead() handler.
> >
> > Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>
> > ---
> >  arch/x86/kernel/smpboot.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index 44c40781bad6..721bb931181c 100644
> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -1416,9 +1416,9 @@ void native_play_dead(void)
> >       play_dead_common();
> >       tboot_shutdown(TB_SHUTDOWN_WFS);
> >
> > -     mwait_play_dead();
> >       if (cpuidle_play_dead())
> > -             hlt_play_dead();
> > +             mwait_play_dead();
> > +     hlt_play_dead();
> >  }
>
> Yeah, I don't think so. we don't want to accidentally hit
> acpi_idle_play_dead().

Having inspected the code once again, I'm not sure what your concern is.

:enter.dead() is set to acpi_idle_play_dead() for all states in ACPI
idle - see acpi_processor_setup_cstates() and the role of the type
check is to filter out bogus table entries (the "type" must be 1, 2,
or 3 as per the spec).

Then cpuidle_play_dead() calls drv->states[i].enter_dead() for the
deepest state where it is set and if this is FFH,
acpi_idle_play_dead() will return an error.  So after the change, the
code above will fall back to mwait_play_dead() then.

Or am I missing anything?